Arcuate printer

ABSTRACT

A printer is disclosed in which a character forming print head is disposed on a carriage which pivots about a central point to describe an arcuate path, adjacent to which is situated an arcuate platen. The media to be marked upon conforms to the arcuate shape of the platen. The print head oscillates or reciprocates from one end of the arc to the other, and back. The arcuate path length may be varied to encompass printing in one or more coplanar stations on an arc in the plane of the print head and pivot.

[451 Dec. 17,1974

United States Patent [191 Cowardin et al.

mo em Adm Cu RK Colombo Wales Delbert C. Thomas, Jr.; William D. Thorne,both of Raleigh, all of NC.

Primary Examiner Edgar S. Burr Assistant Examiner-R. T. Rader Attorney,Agent, or Firm-Edward H. Duffield s e .m my MN k %n no Ur. BA l um .wfir.D. er. m0 IC e n .m s S A M 7 ABSTRACT [22] Filed: June 25, 1973 Aprinter is disclosed in which a character forming print pivots [21]Appl. No.: 373,057

head is disposed on a carriage which about a central point to describean arcuate path, adjacent to which is situated an arcuate platen. Themedia to be marked upon conforms to the arcuate 41 42 shape of theplaten. The print head oscillates or recip- 43; 346/104 rocates from oneend of the arc to the other, and

back. The arcuate path length may be varied to en- Refe en e Cit dcompass printing in one or more coplanar stations on UNITED STATESPATENTS an arc in the plane of the print head and pivot.

1,947,450 Ansley 197/10 5 Claims, 2 Drawing Figures PATENTEL $581 71974BACKGROUND OF THE INVENTION This invention relates to impact printers ingeneral and to serial character or serial dot type of matrix printers inparticular.

PRIOR ART Numerous serial printers exist in the prior art utilizingmoving print heads, drums, disks, belts, etc., but virtually all ofthese are of the linear type in which either the print head or theprinting type fonts are distributed or moved transversely along a linearpath equal to the maximum length of a printed line for the device. Ithas, therefore, been necessary to either provide print heads for eachcharacter station along the line or to provide linear movement for theprinting head, or for a hammer which strikes the print media againstfont characters, depending on the design chosen. This has been aninherent problem with many prior designs since the complexity ofbuilding accurate linear drive apparatus for a moving head entails amechanically cumbersome and expensive lead screw or similar drive systemusing a tension tractor cord, linear rack escapement, etc., to drive acarriage carrying the print head or hammer back and forth. Similarly,and particularly in chain, belt and disk types of printers, high speedprinting has necessarily involved very high speed movement of thecharacter font carrying element or of the print head. This, in turn, hasrequired very sophisticated and accurate timing and positioningmechanisms to provide for quality alignment of the final print andaccurate registration between the moving print element and the medium orfont set as it moves by. These features, too, have been expensive anddifficult to build and maintain. Additionally, the linear type ofprinter has, as a consequence of its design, taken a relatively widehorizontal space always equal to, and usually greater than, the maximumwidth of the printed line, whether there be few or many characters inthe printed line. This is not a desirable attribute in todays trendtoward compactness, lightness, and generally streamlined design inbusiness and commercial data handling equipment. Furthermore, thehorizontal or linear layout of traversing apparatuses in generalrequires such things as line shafts or extended runs of a lead screw ortractor tension cables and numerous pivots, bearings, and miscellaneoussmaller parts at either end of the machinery to support and sustain thetraversing apparatus. In the alternative, where a plurality of hammersor type fonts are utilized with one for each, or for every few,character positions, numerous expensive hammers or character wheels mustbe provided (or their equivalent depending on the type of printercontemplated) which raises the cost and the complexity of the equipmentand requires even more sophisticated timing and coordination between themoving elements of the various mechanisms. High speeds in the mechanicalsense have required extremely fine workmanship and necessarily somegreat expense in designing and manufacturing high quality long-livedequipment to operate under such environmental design criteria as highcharacter print throughput and reliable, low maintenance service. All ofthese problems, and more, are generally those with which printingmachine designers have long been familiar as can be appreciated from thehuge volume of designs and constructions appearing in the prior art.These problemsand others find alleviation in the present invention.

OBJECTS OF THE INVENTION In light of the above and other shortcomingsand deficiencies with the printer designs of the prior art, it is anobject of this invention to produce an improved, more compact serialprinter of simpler and more reliable construction.

It is also an object of this invention to provide a moving head printerwhich is made relatively insensitive to shock and vibration external tothe machine in an improved and simplified manner.

Yet another object of this invention is to improve printing machinedesign by eliminating expensive highspeed bearings, high speed motors,and high power requirements necessary to achieve high-speed printing insome prior designs.

A final object of this invention is to simplify the construction andmaintenance by improving the design reliability and eliminatingunnecessary and unwanted complexity in the structure and in thesynchronization of various coordinated elements making up the printer.

SUMMARY OF THE INVENTION The foregoing objects and others are. met byproviding a serial dot or serial character print head with a rotating orarcuately traveling carriage. The carriage pivots about a central point,and printing occurs at the outer periphery or arc described by themoving print head against a curved platen backing up the media to beprinted upon. The print line length so provided, is equal to a muchgreater horizontal run and can be confined to lie within a horizontaldimension less than that of the total line length. The invention alsomakes possible the counter balancing of the print head to make itinsensitive to shock and vibration introduced into the framework fromoutside the printer itself. Simplified position sensing and reducedpower requirement are also realized due to the direct rotational motionbetween the main power drive input and the print head motion output. Thereduced speed requirement which comes from having to drive the main massof machinery at a relatively slow rotational speed while the printinghead tip or print element traverses the print line at a relativelyhigher linear velocity also greatly aids the power reduction. Thesefactors and others reduce the total complexity of the unit. Because ofthe relatively low speeds and stresses involved, the type of bearings,motors, clutches, and other mechanical elements can be greatlysimplified and made of reduced cost materials without sacrificingperformance or maintainability over a long life.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 illustrates a partially cut-awaypictorial view, in simplified form, of the preferred embodiment of theinvention and clearly shows the major working elements.

FIG. 2 illustrates, in cross section, a preferred motion detector andquantizer as used in the preferred embodiment.

Turning now to FIG. 1, the basic elements of a preferred embodiment ofthe invention, their arrangement and mode of operation will bediscussed. In FIG. 1, the

main pivot shaft 1 is shown in a partially cut-away view in itsvertically mounted position. Shaft 1 is driven by a gear 2 which is, inturn, driven from the gear 3 mounted on the end of a shaft connecting itto a speed reduction gear box connected to a drive motor 4. Motor 4turns continuously and provides output power through gear 3 to drivegear 2 at an angular velocity equal to that desired for the printingsweep. Gear 2 turns freely on shaft 1 when it is not engaged by a clutch5 to apply power to shaft 1. Clutch 5 is not shown in detail in thefigures but a preferred form of clutch 5 may be seen in the IBMTechnical Disclosure Bulletin, Vol. 14, No. 2, July, 1971, pages 455-456which, for purposes of description of a suitable clutch system for thepreferred embodiment, is made a portion hereof. The relatively slowrotating input from gear 2 is coupled by clutch 5 to the shaft 1 bypulling on the end of a spiral gripping spring in the clutch 5 which isof a type well-known and utilized in many spring clutches. A solenoid,not shown, is energized when print head motion is desired. The solenoidpulls on the end of the spring and causes engagement by wrapping of thespring onto the shaft and holds the clutch engaged until the solenoid isreleased. By the means of clutch 5, the rotary input motion of gear 2 isconnected to shaft 1 and to the elements mounted on shaft 1 to causethem to rotate at the angular velocity of gear 2.

Main support carriage 6 is mounted on the top of shaft 1 in a fixedorientation so that it will move whenever shaft 1 moves. Carriage 6supports print head 7 and counterweight 8. The print head 7, which inthe preferred embodiment is a wire matrix print head similar to the typeshown in U.S. Pat. Nos. 3,108,534, 3,592,3ll and 3,672,482, (which forthe purposes of describing an operative print head, are made a parthereof,) is disposed in a fixed position on carriage 6 and is held inplace for example, by screws 9 on either side of the print head housing.Nose portion 10 of print head 7 is placed in close proximity to theplaten or platens ll, several of which are illustrated disposed around acircumferential path and in horizontal coplanar alignment with the nose10 of print head 7. Arcuate platens 11 are supported in the main frame12 in a fixed position during printing as illustrated. Platens 11 may belonger or shorter, more numerous or fewer than those platens 11illustrated.

It is not essential that a wire matrix print head of any specific typebe utilized as it is well-known that serial dot wire matrix print headsand serial whole character wire matrix print heads can be built and areavailable which could be equally utilized to advantage in the presentembodiment of the invention. Similarly, electro discharge heads forprinting on electrically sensitive paper, such as is well-known in theart, or similarly wellknown heat sensitive paper and hot wire matrixdevices could easily be utilized in place of the preferred wire matrixprint head.

In FIG. 1, three printing stations A, B, and C have been illustrated asutilized in the preferred embodiment to provide multiple copies of thesame data. This feature is desirable for use, for example, in a retailstore environment where customer purchase receipts, a store record orjournal list maintained in the machine, and a special document printingor franking station, (for example, for imprinting credit slips orchecks) are all required to have information printed on them. -A papertape or document sheet 13 is shown at station B and would be wound ontoa take-up roll 14 by suitable mechanisms not illustrated, with paper 13being supplied from a supply roll not shown in FIG. 1. The document 13is driven by suitable means such as driving the take-up roll by a shaft15 or by friction wheel drive 16 illustrated in printing stations A andC. Friction wheels 16 bear against one surface of paper or document inthe station and force it upward or downward when the wheels are turnedby suitable means such as a stepping drive or continuously turning motordriving a clutch system.

The backup platens 11 are preferably of a hard steel, such as Cu-Nisintered steel of 7.1 specific gravity, but if another type of printinghead such as an electro discharge head were utilized, it should beunderstood that cooperating platen of suitable design would be placed inposition instead of the backup platen l l illustrated. For purposes ofdescription, a dotted arrow 17 has not been illustrated on the top ofcounterweight 8 to show the direction of motion to be expected whenclutch 5 is engaged. In this example, the counterweight and print headattached to carriage 6 would move in the counter clockwise directionupon energization of the clutch 5. Counterweight 8 offsets the mass ofthe overhung carriage 6 and of the print head apparatus born on carriage6 so that the totality of the elements attached to the end of driveshaft 1 are dynamically balanced against unwanted vibration or motiondue to external shocks and movements. Print head 7 will, therefore,maintain a steady and constant speed whenever clutch 5 is engaged.Printing, as is well-known to those familiar with any of the currentlyknown wire matrix print heads, is accomplished by driving one or more ofa plurality of fine wires outward against either a pressure sensitivedocument or against an ink carrying (or similar) marking material ribbonand against the document. An alternative arrangement is to provide aninked platen 11 or other marking means disposed behind the documentrelative to the print head so that back printing is accomplished. In thepreferred embodiment, an ink impregnated ribbon is stored in a cassette(not shown) which is mounted on top of counterweight 8. The cassette isutilized to feed a ribbon out of the cassette around the nose 10 ofprint head 7 and back into the cassette for storage with suitable drivemeans being provided to withdraw the ribbon from the cassette at oneside and stuff it into the cassette at the opposite side. A typicalribbon cassette of this type which may be utilized in the preferredembodiment is shown in U.S. Pat. application, Ser. No. 209,684, filedl2,-20,-7I.

As print head 7 moves counter clockwise, it scans an arcuate path onwhich are disposed the various print stations A, B, and C as areillustrated. A continuous print path and continuous platen could beused, if desired. Some means must be provided for notifying theelectronic system providing the wire matrix wire driving pulses that aspecific location for printing has been reached.

Such a means is shown in FIGS. 1 and 2 and consists of an angular shaftposition sensor or motion quantizer 18 which contains a light source 24and a photo sensor 25 disposed on opposite sides of an interposer l9having numerous apertures 20 and 21 disposed about its periphery. Theuse of such optical position sensing systems is well-known in the artand need not be discussed in great detail. The theory of operation isthat light impinging on one side of interposer 19 shines throughapertures as they move between the source of light and a photo sensordisposed on the opposite side of the interposer. Each time such anaperture passes between the source of light and the photo sensor, anelectrical pulse is produced. These pulses, as indicia of quantizedmotion, may be counted by suitable counting means to determine, inrelative numbers of steps corresponding to the placement of theapertures on the interposer 19, the relative position of the printinghead along its printing path to any desired degree of accuracy.Equivalent systems may utilize mark sensing optical means instead oflight transmitting optical means, magnetic mark sensors, mechanicalbrushes or the like. It is, of course, well-known that either the sourceand sensor may move with the interposer fixed, or vice versa, since itis relative motion between the elements that is sensed. A preferredembodiment, however, utilizes a perforated interposer 19 carrying aplurality of finally spaced apertures 20 for producing pulsescorresponding to character width distances traversed by the head.Apertures 21, of somewhat larger size and of widely disposed location,are utilized to provide information for block positioning such as, inthe preferred embodiment illustrated, notification to the system thatthat print head is located opposite the beginning of. a new scan atstation A, station B or station C. The block position signals correspondto the notches 21 and the large blank area 22 taken from the peripheryof interposer 19.

Some means must also be provided for returning the print head andcarriage back to the home or start position by reversing the rotation ofdrive shaft 1 when printing has been completed at the last station atwhich printing was desired. In the preferred embodiment, this functionis satisfied by the provision of a helical return spring 23 which storesenergy as drive shaft 1 is coupled by clutch 5 to the gear 2 to drivethe print head around its printing path in a counter clockwisedirection. Then, when clutch 5 is finally released at the end ofprinting, spring 23 utilizes the energy stored in it to return and printhead and counterweight by driving shaft 1 in the clockwise direction.Excessive rebound forces are taken up by providing a suitable dash potor other damper means 24 in a position to contact carriage 6 as itreturns to the home position opposite the end of station A.

Suitable anti-friction bearings 25 are also included at either end ofthe drive shaft 1 so that free rotation of the balanced system describedcan be attained.

Turning now to FIG. 2, a more detailed description of the positionsensor 18, the interposer 19 and the apertures 20 and 21 will beprovided. As stated previously, the interposer 19 is attached at one endof rotating main shaft 1 while the matrix print head 7, attached to itscarriage 6, is rigidly affixed to the opposite end of shaft 1. Shaft 1was chosen for the attachment point of the interposer 19 because itsangular rotation is relatively slow and because the print head 7 isfirmly attached to the other end of the shaft, thus eliminating printposition variations due to tolerances in the drive train and variationsin the motor speed which could cause misreading of the exact headposition relative to the position of the interposer 19. interposer 19has apertures that can be termed a grid and, in the preferredembodiment, the grid is made of etched metal, although it could be madeof plastic, photographic film,

or any other material opaque to light. Similarly, if a lighttransmissive grid system is not desired, reflective material such asused by mark sensing means of an optical type or magnetic material suchas used by mark sensing means of the magnetic type could be easilysubstituted in place of the transmissive photosensor system. Asillustrated in FIG. 1, the grid on interposer 19 rotates in an arc ofslightly over as the head accesses the three separate print stationsshown. The actual linear velocity at the tip 10 of the matrix print headis approximately 8.8 inches a second. The surface velocity at the gridon interposer 19 is somewhat less because it is at a shorter radius fromthe center of shaft 1 than the tip 10 of print head 7.

The grid portion of interposer 19 has three groups of 30 slits each witha nominal width per slit of 0.02 inches. The slits just named aredenominated 20 in the figures and each slit corresponds to a timing markfor the beginning of a character printing cycle. These slits areapproximately 0.042 inches apart and are placed at a three inch radiusfrom the center of shaft 1; the actual character spacing produced at theprint head is 0.083 inches because of the greater radius at the tip ofthe print head which has approximately a 3 inch greater radius (a totalof 6 inch radius approximately), and the grid was chosen at a 3 inchradius smaller than the radius of the print head tip to minimize weightand torque problems when the rotating shaft was returned to the restposition.

A second series of notches or slits 21 and an open space 22 on theinterposer l9 determine the home position and the end of line positionfor each of the three print stations A, B, and C. This section of thegrid is placed at the outer periphery of the interposer 19 and isradially outside the character position grid of apertures 20. The usingor host electrical system requires that a home position be distinguishedfrom the three end of line positions in this embodiment. The notches 21and the open space 22 are made so that two photo detectors 25, one forthe grid of apertures 20 and one for notch or space 21, or 22, areactivated only at the home position so that the home position can bedistinguished from the three end of line positions indicated by notches21. Thus, pulses are produced due to the movement of the interposerwhich corresponds to each character spacing position and a separatepulse is produced by notches 21 and finally, a double pulse is producedby a proper placement of a final notch 21 and one of the apertures 20 sothat two detectors will be on simultaneously when the head is in thehome position illustrated in FIG. 1.

Photo detector 18 is a commercially available device (such as part No.08-592 5-060, made by HE! Corp. of Chaska, Minn.) which has two lightemitting diodes 24, specified as spaced .02 apart, which could as wellbe lamps, positioned on one side of a U-shaped block of material 18 andfacing towards the opposite leg of the U. On the opposite side of the Uis positioned a lense 26 to focus light from the light emitting diodeonto a photo diode or photo cell which produces electrical pulseswhenever light from the light emitting diode source passes through theinterposer either through a slit or through one of the notches, and isfocused by the lense onto the sensor. The sensor gives a charactertiming pulse to the using host system for every 47 minutes and 45seconds of arc traveled at the aforementioned radius and slit size. Thisare corresponds to approximately one pulse every 9.4 milliseconds at thespeed of travel utilized in the preferred embodiment. These pulses arefed over an interface line to the controlling system which will thengenerate specific wire firing signals at every 0.9 milliseconds togenerate proper characters by the repeated firing of the wires in thewire matrix print head 7 between character impulse signals from thedetector 18. This, of course, assumes that a character is to be printed;the system connected to the printer will determine whether or not at agiven character impulse signal from the detector 18, any character is tobe printed at all.

MODE OF OPERATION Although it has not been illustrated in the figures inany detail, it will be understood by those familiar with wire matrixprinters, and printers in general, that the signals for causing thegeneration of dot matrix charac ters must be provided by some kind oflogic system at the proper time and in the proper sequence to print outcharacters as desired. These systems are well-known in the art and donot form a part of the present invention which is directed only towardsthe printing or marking mechanism itself which produces the propermotions and signal pulses from which the using system can derive timingsfor the application of electrical currents to the wire firing circuitswhich drive the wire drivers in the print heads. Two modes of operationare generally possible in wire matrix printers: characters may beprinted on demand and one at a time or, as is more usually the case andgenerally more economical, a plurality of characters, usually enough tomake up a line of printing, are stored in a buffer or electronic storagedevice until a full load of data for printing has been collected. When aload of data for printing has been collected and the print head issensed to be in the home position, control electronics logic systemswill begin emptying the contents of the buffer and simultaneouslytherewith will engage the print head drive system by energizing theclutch so that as each character position pulse is produced by theposition sensor 18, the specific wire firing pattern to generate thedesired character then exiting from the buffer can be produced andapplied to the wire matrix print head driving circuits to generate thewire impacts for forming the character desired.

In the present embodiment, the return of the print head to the homeposition energizes two photo sensors as previously discussed whosecombined and simultaneous signal is utilized by logic systems having acontrolling system to which the printer is attached to signal that theprint head is returned to the home position and is ready to beginprinting a new line of data on command. Data characters to be printedare then transmitted in serial dot form for each character as the printhead moves and the sensor 18 detects the beginning of character printingpositions as previously described. In the specific embodimentillustrated, the, same data is printed at one, two, or three stationsand, in the normal mode of operation will print at stations A and B eachtime and station C only when a document to be printed upon has beeninserted by hand into the printing area. Inserting such a document byhand in the station C closes a microswitch (not shown) to indicate tothe system that a document is in place and that the head should betraversed the full width of its printing path to cause printing instation C as well. When no document is in station C, the clutch isdisengaged upon sensing of the second end of line signal by sensor 18and the return spring 23 will return the printing head to the homeposition.

ADVANTAGES Many advantages accrue from the specific design of thepreferred embodiment of the invention. First, the bearings utilized insupporting the main shaft 1 and, although not shown, the shaft on whichdrive gear 3 is mounted, and further, in the reduction unit suppliedwith motor 4, all may be of an inexpensive, nonprecision type dueprimarily to the low stresses imposed on them due to the nature of thedesign. Specifically, since the motor is continually operating, loads ofgreat magnitude are not presented and stresses are basically low at themotor shaft and throughout the gear train because the matrix print head7 is relatively light in weight and is counter balanced by counterweight8 so that only the rotational inertia of the system must be overcome inaddition to the slight rewind torque necessary to store up sufficientenergy in spring 23. The output speed delivered by gears 2 and 3 toshaft 1 through clutch 5 would, if the print head mechanism were allowedto rotate through a full 360, be at a speed of only about 13 RPM andthus the main bearings 25 which support shaft 1 need not be of a specialhigh speed or heavy duty design.

Additionally, motor 4 need not be a large size motor since, at thereduced RPM output finally transmitted to shaft 1, very little force isrequired in the form of torque to accelerate the print mechanism and tostore energy in spring 23.

Similarly, too, clutch 5, although it is of a specialized design aspointed out in the referenced IBM Technical Disclosure Bulletin, has lowwear characteristics due to the fact that little slippage is experiencedand stresses imposed on the clutch parts are quite low due to the lowspeed and the low force requirements produced in accelerating the printhead and mechanism as described.

The power requirement for producing a traverse, or one full oscillation,of the print head from home position to end of travel and home again isprobably similar to that required in a linear traversal printing headsystem, but the duty cycle is less stringent in time requirementsbecause of the low speed motions at the output of the mechanicam systemand because of the simplified return spring type of return drive. Thesystem is, of course, more compact than the equivalent linear systemwould be to print in three adjacent stations because the traversingapparatus required in a linear system and the necessary side supportsand bearings have all been eliminated. In addition, because the paper ordocuments fed through the printing stations take on the arcuate form ofthe platen, the paper develops a column strength due to its arcuatecurvature transverse to a direction of feeding the paper. This allows amuch simpler feeding mechanism to be utilized since the inherent columnstrength and stiffness of the paper are enhanced by the arcuatecurvature imparted to it in the area of printing.

In addition, the print head and photo sensor relationship is a directone-to-one motion control which, due to the fact that the positionsensor is fixed and the interposer and the print head are both affixedat relative spacing to the ends of a rigid shaft,'creates an exceedinglyaccurate position sensing device which is not affected by backlash andmanufacturing tolerances. Adjustment of the relative position of theinterposer and the print head may be accomplished simply by means of aset screw affixing either element to the drive shaft 1.

Because of the relatively smaller size of the printer, the low powerrequirement and the simplicity of structure which is made possible bythe arcuate platen and reciprocating, arcuately moving print head, theoverall weight and expense of the printer and the ease of itsmanufacture are greatly improved over previous designs. Additionally, noexpensive and hard to maintain stepping or incrementing motions arerequired which require complicated mechanisms and more complicated andexpensive electronic systems. Infinite character printing ability withinthe limits of the dot matrix resolution is inherently provided by thematrix type printers, but the ability to produce a device in which thematrix type of print head is virtually insensitive to outside inducedshocks and vibrations because of the balanced nature of thereciprocating rotary drive mechanism make for an improved performance ata relatively lower cost.

While the invention has been particularly shown and described withreference to a preferred embodiment thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the spirit and scope of theinvention.

What is claimed is:

1. Media marking apparatus, comprising:

at least one mark producing means for making visible marks on a medium;

support means for supporting said mark producing means in proximity to amedium to be marked and in a position to make a mark when said markproducing means is operated;

pivot means mounted at a fixed radial distance from said medium andsupported in bearings for rotation about the central axis of said pivot,said axis being perpendicular to the radius establishing said radialdistance;

said support means supporting said mark producing means being rigidlyaffixed to said pivot for rotation therewith to traverse an arc of acircle having its center on the axis of said pivot;

at least one arcuate platen means for supporting said medium at saidfixed radial distance from said pivot in a position to be marked upon bysaid mark producing means while it traverses said arc, said arc of saidplaten means being cylindrical and having its axis coincident with saidaxis of said pivot means;

drive means connectable to said pivot means for turning said pivotthrough an arc;

return drive means coupled to said pivot means for turning said pivotmeans in an arc in an opposite direction to that produced by the actionof said drive means when it is connected to said pivot means; and

motion sensing and quantizing means for indicating the rotation of saidpivot and for signalling the relative amount of travel along said arctaken by said support means and said mark producing means in response tosaid drive means.

2. Marking apparatus as described in claim 1,

wherein:

said mark producing means comprises a wire matrix printing mechanism;and

said return drive means comprises a resilient, stressed helical springconnected at one end to a fixed point relative to said pivot and at theother end connected to said pivot and encircling it, thereby applying areturn torque to said pivot.

3. Apparatus as described in claim 2, further comprising:

a counterweight affixed to said support means in a position to counterbalance said printing mechanism and said support so that the center ofgravity of the assemblage so produced lies approximately on the centralaxis of said pivot.

4. Apparatus as described in claim 3, wherein:

a plurality of said arcuate platens for supporting a like plurality ofmarkable media are placed at adjacent quantized distance locations alongsaid arc; and

said motion sensing and quantizing means comprises a source of light, asensor positioned relative to said source of light, a sensor positionedrelative to said source of light to receive light therefrom, and aninterrupter means having alternate light transmissive and opaque areas.

5. Apparatus as described in claim 4, wherein:

said source and sensor are affixed in a stationary position; and

said interrupter is adapted to be moved by the motion of said pivot tointerrupt the flow of light from same.

1. Media marking apparatus, comprising: at least one mark producingmeans for making visible marks on a medium; support means for supportingsaid mark producing means in proximity to a medium to be marked and in aposition to make a mark when said mark producing means is operated;pivot means mounted at a fixed radial distance from said medium andsupported in bearings for rotation about the central axis of said pivot,said axis being perpendicular to the radius establishing said radialdistance; said support means supporting said mark producing means beingrigidly affixed to said pivot for rotation therewith to traverse an arcof a circle having its center on the axis of said pivot; at least onearcuate platen means for supporting said medium at said fixed radialdistance from said pivot in a position to be marked upon by said markproducing means while it traverses said arc, said arc of said platenmeans being cylindrical and having its axis coincident with said axis ofsaid pivot means; drive means connectable to said pivot means forturning said pivot through an arc; return drive means coupled to saidpivot means for turning said pivot means in an arc in an oppositedirection to that produced by the action of said drive means when it isconnected to said pivot means; and motion sensing and quantizing meansfor indicating the rotation of said pivot and for signalling therElative amount of travel along said arc taken by said support means andsaid mark producing means in response to said drive means.
 2. Markingapparatus as described in claim 1, wherein: said mark producing meanscomprises a wire matrix printing mechanism; and said return drive meanscomprises a resilient, stressed helical spring connected at one end to afixed point relative to said pivot and at the other end connected tosaid pivot and encircling it, thereby applying a return torque to saidpivot.
 3. Apparatus as described in claim 2, further comprising: acounterweight affixed to said support means in a position to counterbalance said printing mechanism and said support so that the center ofgravity of the assemblage so produced lies approximately on the centralaxis of said pivot.
 4. Apparatus as described in claim 3, wherein: aplurality of said arcuate platens for supporting a like plurality ofmarkable media are placed at adjacent quantized distance locations alongsaid arc; and said motion sensing and quantizing means comprises asource of light, a sensor positioned relative to said source of light, asensor positioned relative to said source of light to receive lighttherefrom, and an interrupter means having alternate light transmissiveand opaque areas.
 5. Apparatus as described in claim 4, wherein: saidsource and sensor are affixed in a stationary position; and saisinterrupter is adapted to be moved by the motion of said pivot tointerrupt the flow of light from same.